Cleaning processes using hydrofluorocarbon and/or hydrochlorofluorocarbon compounds

ABSTRACT

Disclosed are methods of cleaning articles of manufacture using hydrofluorocarbon and hydrochlorofluorocarbon fluids. The methods comprise generally the steps of (a) providing a hydrofluorocarbon and/or hydrochlorofluorocarbon fluid in liquid or supercritical state; (b) contacting an article of manufacture with said fluid; and (c) removing substantially all of said fluid from said article of manufacture.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a divisional of U.S. application Ser.No. 09/670,127 (pending) which was filed with the United States Patentand Trademark Office on Sep. 26, 2000, and which claims priority to U.S.Provisional Application Ser. No. 60/162,554, which was filed with theUnited States Patent and Trademark Office on Oct. 29, 1999. Both ofthese applications are herein incorporated by reference.

FIELD OF INVENTION

[0002] The present invention relates generally to the use ofhydrofluorocarbon and hydrochlorofluorcarbon compounds in cleaningprocesses. More specifically, the present invention relates to a methodof cleaning a wide variety of articles of manufacture, especiallyprinted circuit boards, using hydrofluorocarbons and/orhydrochlorofluorocarbons in certain subcritical states and insupercritical states.

BACKGROUND

[0003] In the manufacture of printed circuit boards, surface-mountassemblies and other, similar, electronic assemblies, solder flux iscommonly applied to the solderable surfaces of the assembly prior tosoldering. Solder flux is usually a blend of rosin (abietic acid) andactivators. The activators are typically ionic compounds such as aminehydrochlorides. The purpose of the flux is to remove metal oxides fromthe surface of metal leads in order to render them more solderable.During the soldering operation the flux is heated to very hightemperatures (essentially the temperature of molten tin/lead solder) andis “baked” onto the assembly. This solder flux residue is potentiallyharmful to the assembly and must be removed. The ionic materials, whenexposed to moisture, are particularly harmful as they become mobile andconductive.

[0004] In the recent past, chlorofluorocarbon (CFC) solvents were usedto clean solder flux from articles of manufacture such as printedcircuit boards. These solvents were very effective and economical.However, since the recognition that CFC's are harmful to theenvironment, many manufacturers have switched to aqueous cleaners. Suchcleaners clean adequately but are disadvantageous because they requireenergy intensive processes and costly disposal after use.

[0005] Carbon dioxide, both in the liquid state and in the supercriticalstate, has been proposed as an alternative to aqueous cleaning. However,the use of carbon dioxide is disadvantageous for several reasons. Onesuch disadvantage is that the use of carbon dioxide requires very highpressures, which necessitate the use of relatively expensive pressureequipment. Another disadvantage is that carbon dioxide is not effectivein cleaning some polar substrates.

[0006] Hydrofluorocarbons (HFC's) are an environmentally saferalternative to CFC's but it has been reported in the art that they areineffective cleaners. Leblanc, for example, notes that “though they[HFC's] contain no chlorine, their high vapor pressure and lowsolubility make them poor cleaners.” [“The Evolution of CleaningSolvents, Precision Cleaning, May, 1997, p. 14.] This is supported byKanegsberg who comments that HFC's, “while useful as carrier for othersolvents and as rinsing agents, with a Kauri Butanol (KB) number ofabout 10, on their own have little or no solvency for most soils ofinterest”. [“Precision Cleaning Without Ozone Depleting Chemicals”,Chemistry & Industry, October, 1996, p. 788.]

[0007] Recognizing these and other drawbacks of the prior art, thepresent inventors have perceived a need for a new, efficient and moredesirable method for cleaning articles of manufacture, such as printedcircuit boards. These and other objects are achieved by the presentinvention as described below.

DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

[0008] The present invention is directed to a method of cleaning a widevariety of articles of manufacture, but especially printed circuitboards, using hydrofluorocarbon (“HFC”) compounds orhydrochlorofluorocarbon (“HCFC”) compounds, or mixtures of HFC and HCFCcompounds. The present inventors have found unexpectedly that certainHFC's and HCFC's alone or in combination with small amounts of polarsolvents dramatically outperform carbon dioxide in liquid andsupercritical based cleaning processes. Since the HFC's and HCFC's haveboth lower vapor pressures and critical pressures than carbon dioxide,the required operating pressures for equipment needed for the presentmethods will be lower, and the equipment with which they are used willbe less expensive. This is important since one of the barriers to theuse of supercritical technology is the high cost of very high pressureequipment.

[0009] According to certain embodiments of the present invention, themethods of cleaning an article of manufacture containing contaminants ona surface thereof comprise the steps of (a) providing a hydrohalocarboncompound as a supercritical fluid; (b) contacting at least thecontaminated surface of the article of manufacture with saidsupercritical hydrohalocarbon-containing fluid; and (c) removingsubstantially all of said supercritical hydrohalocarbon-containing fluidfrom said article of manufacture. As used herein, the termhydrohalocarbon refers to any compound that is an HFC or an HCFC.

[0010] It is believed that the methods of the present invention can beused to clean a wide variety of articles of manufacture including, forexample, printed circuit boards, including circuit boards with surfacemounted devices, and silicon wafers. For silicon wafers, the presentinvention can be used to effectively clean and remove cured and uncuredphotoresists, which are deposited on said wafers when patterns for themanufacture of computer chips are laid down thereon. Moreover, it isbelieved that the methods of the present invention are especiallyapplicable to the cleaning of printed circuit boards.

[0011] As used herein, the term “supercritical fluid” refers generallyto a fluid, comprising at least one HFC or HCFC compound, in itssupercritical state and also refers to HFC or HCFC compounds which arein the liquid state and have a boiling point below about 16° C. atatmospheric pressure. The present inventors believe that such liquidfluids act in certain respects like fluids in their supercritical statefor many embodiments of the present invention. Any fluid in asupercritical state comprising a single HFC or HCFC fluid or mixture ofHFC and/or HCFC fluids, as well as any mixture of HFC or HCFC fluid(s)with other materials and/or additives will be a “supercritical fluid”for purposes of the present invention. Any liquid-state fluid, having aboiling point below about 16° C. at atmospheric pressure, comprising asingle HFC or HCFC fluid or mixture of HFC and/or HCFC fluids, as wellas any mixture of HFC or HCFC fluid(s) with other materials and/oradditives will be a “supercritical fluid” for purposes of the presentinvention.

[0012] Any of a wide range of HFC's or HCFC's can be used in the methodof the present invention to provide supercritical fluids. Suitable HFC'sinclude, for example, difluoromethane (“R-32”), pentafluoroethane(“R-125”), isomers of tetrafluoroethane (“R-134 and R-134a”),trifluoromethane (“R-23”), trifluoroethane isomers (“R-143” and“R-143a”), pentafluoroethane (“R-125”) and isomers of pentafluoropropane(for example, “R-245fa”, “R-245ea”, “R-245ca”, and “R-245eb”) and thelike. Suitable HCFC's include chlorodifluoromethane (“R-22”),tetrafluorochloroethane (“R-124”) and the like. In certain preferredembodiments, the HFC used in the present invention is R-23 or R-32. Inother preferred embodiments, the HCFC used in the present invention isR-22 or R- 124.

[0013] The HFC and HCFC compounds of the present invention can be usedalone or in admixture with one another. When mixtures are used,azeotropes or azeotrope-like or constant boiling mixtures containing thecompounds are particularly preferred. For example, such mixtures includethe azeotrope of R-32 and R-125 as described in U.S. Pat. No. 4,978,467,incorporated herein by reference.

[0014] Other materials can be added to the hydrohalocarbon-containingfluids to improve their cleaning ability. These materials include:straight chain, branched and cyclic alcohols having from about one toabout eight carbons, such as methanol, ethanol, propanol and butanol;straight chain, branched and cyclic chlorinated alkanes such aschloroform, methylene chloride, chlorinated ethanes, propanes andbutanes; oxygenate compounds such as aldehydes, ketones, esters andethers, such as dimethyl ether; compounds that have heteroatoms such asnitrogen, sulfur and phosphorous; and aromatic and non-aromatic cycliccompounds. Generally, these materials are present in an amount of fromabout 0.1 wt % to about 25 wt % and preferably in an amount of fromabout 0.1 wt % to about 10 wt %.

[0015] In certain preferred embodiments, from about 0.1 wt % to about 10wt % of methanol is added to a hydrohalocarbon-containing fluid,especially one consisting essentially of an HFC. Applicants have foundthat the addition of methanol in such amounts tends to increase thecleaning efficiency of supercritical fluids provided fromhydrohalocarbon compounds. For example, the use of about 0.5 wt % ofmethanol with a supercritical fluid consisting essentially of R-125 wasfound to be six times more efficient at cleaning according to thepresent invention than R-125 neat.

[0016] Additives known to be useful in solvent cleaning applications mayalso be used in the process of the invention. Suitable additivesinclude: stabilizers such as nitroalkanes having from about one to aboutfive carbons; acid neutralizers such as alkyl and aromatic epoxides; andsurfactants such as ionic or non-ionic surfactants, includingfluorinated and non-fluorinated surfactants. These additives aregenerally present in an amount of from about 0.1 wt % to about 5 wt %and preferably from about 0.1 to about 1 wt %.

[0017] According to certain embodiments of the present invention, theproviding step (a) comprises subjecting a fluid comprising an HFC and/orHCFC to conditions sufficient to form a supercritical fluid. Generally,the HFC's, HCFC's, other materials and additives described herein areall commercially available. HFC or HCFC fluids suitable for use assupercritical fluids in the present invention are formulated, accordingto certain embodiments, by mixing the desired ingredients undersufficient pressure to put the HFC or HCFC in a liquid state. Forembodiments in which the normal boiling point of the compound is aboveabout 16° F., the liquid state hydrohalocarbon(s) is (are) subjected toconditions sufficient to put the fluid in a supercritical state. Forfluids in the supercritical state, the temperature and/or pressure maybe raised above the critical temperature and critical pressure,respectively, of the fluid. For example, FIG. 1 is a block diagram of anapparatus 10 for use with the present invention in which a fluid may besubjected to conditions sufficient to form a supercritical fluid. InFIG. 1, a HFC/HCFC fluid container 11 is connected to a pump 12. Pump 12is connected to chamber 13 in which is contained an article 14 to becleaned. According to the present method, an HFC/HCFC fluid, not shown,held in container 11 is passed through pump 12 wherein the pressureand/or temperature of the fluid are raised to a point sufficient to forma supercritical fluid. The supercritical fluid is then passed intochamber 13 for cleaning the article contained therein. This method andany other known methods for providing a supercriticalhydrohalocarbon-containing fluid and then exposing a contaminatedarticle of manufacture to said supercritical fluid may be used accordingto the present invention.

[0018] Those of skill in the art will be readily able to determine thetemperature and pressure to which HFC and HCFC compound must besubjected to form supercritical fluids according to the presentinvention. The temperature will depend, in part, on both the HFC'sand/or HCFC's used and upon the pressure. In general, supercriticalfluids are provided according to the present invention by maintainingthe HFC or HCFC at a temperature of from about −10° C. to about 200° C.,and even more preferably from about 15° C. to about 130° C. In certainpreferred embodiments, the pressure at which the supercritical fluid isprovided is generally from about 25 psia to about 10,000 psia, and morepreferably from about 100 to 3,500 psia.

[0019] In light of the above disclosure those of skill in the art willbe readily able to provide a supercritical fluid according to thepresent invention without undue experimentation.

[0020] According to preferred embodiments of the present invention, thecontacting step (b) comprises covering at least a portion of acontaminated surface of an article of manufacture with a supercriticalfluid and dissolving at least a portion of the contaminants from thesurface in the supercritical fluid such that removal of the fluidresults in the removal of contaminants from the contacted portion of thearticle of manufacture.

[0021] Any known method for covering at least a portion of acontaminated surface of an article of manufacture can be used in thepresent invention including, for example, immersion of the article inthe supercritical fluid, spraying the article or the like. In preferredembodiments of the present invention, the coating method is immersion.

[0022] Any known method for immersing an article of manufacture in asupercritical fluid can be used in the present invention including,especially, immersing an article of manufacture in a stream ofsupercritical fluid. For example, the cleaning apparatus illustrated viablock diagram in FIG. 1 can be used to immerse an article of manufacturein a stream of supercritical fluid according to preferred embodiments ofthe present invention. In FIG. 1, a supercritical fluid, formed bypassing an HFC or HCFC fluid through pump 12, is allowed to stream intochamber 13 and out through an expansion valve 15 into a low-pressurecollection vessel 16. When introduced into chamber 13, the supercriticalfluid expands to fill the chamber. Consequently, the article 14 heldwithin chamber 13 is immersed within the supercritical fluid stream asit flows from pump 12 to vessel 16 via valve 15.

[0023] Alternatively, the article of manufacture may be immersed via“pressure pulsing” as described in U.S. Pat. No. 5,514,220, issued toWetmore et al., incorporated herein by reference. Generally, pressurepulsing involves contacting an article with a fluid under supercriticalor near supercritical conditions and periodically pulsing or spiking thepressure of the supercritical fluid to higher levels and then loweringthe pressure back to the original level. Those of skill in the art willbe readily able to adapt such a procedure to the present inventionwithout undue experimentation.

[0024] In embodiments of the present invention wherein the supercriticalfluid is a fluid in its liquid state, an article of manufacture may beimmersed by dipping at least a portion of the article of manufacture inthe liquid-state fluid. Those of skill in the art will be readily ableto adapt such a procedure to the present invention without undueexperimentation.

[0025] Those of skill in the art will recognize that the conditions,including flow rate, temperature, pressure and time period, under whichan article is immersed in a stream of supercritical fluid according topreferred embodiments of the present invention will vary depending on anumber of factors including the HFC or HCFC fluids used. For example, incertain preferred embodiments wherein the HCFC fluid comprised R-22,100% of solder flux impurities on a printed circuit board were removedat a temperature of about 25° C. and a pressure of about 151 psia.Additionally, in certain preferred embodiments wherein the HFC fluidcomprised R-32, 90% of solder flux impurities on a printed circuit boardwere removed at a temperature of about 100° C. and a pressure of about1200 psia. In light of the above disclosure, one of ordinary skill inthe art will be readily able to optimize immersion conditions for use inthe present invention without undue experimentation.

[0026] The removing step (c) preferably comprises evaporating thesupercritical fluid. According to preferred embodiments, the evaporatingstep comprises changing the pressure and/or temperature of thesupercritical fluid such that the fluid is converted to the gaseousstate. As those of skill in the art will recognize, fluids in theirsupercritical state can be readily converted to the gaseous state bychanging the pressure, the temperature or both pressure and temperaturesuch that the fluid is no longer under supercritical conditions.Furthermore, fluids of the present invention in their liquid state(fluids whose atmospheric boiling point is about 16° C. or lower) cangenerally be converted to gasses by lowering the pressure or increasingthe temperature.

[0027] The change in conditions surrounding the supercritical fluids cancause contaminants dissolved in the supercritical fluid to precipitateout of solution. Accordingly, in certain preferred embodiments, after anarticle has been contacted by a supercritical fluid to removecontaminants therefrom, the supercritical fluid is passed into acollection chamber wherein the pressure and/or temperature is changed todrop the contaminants out of the fluid. For example, in FIG. 1, anarticle 14, held within chamber 13 is immersed in a stream ofsupercritical fluid. The supercritical fluid removes contaminants fromarticle 14 and flows through valve 15 into collection vessel 16. Whenthe fluid passes through valve 15, the pressure of the fluid is dropped,and the supercritical fluid is converted to a gas in collection vessel16. The contaminants dissolved within the fluid are precipitated andcollected within vessel 16. In light of the above disclosure, those ofskill in the art will be readily able to evaporate and removesupercritical fluid according to the present invention.

[0028] After a supercritical fluid has been evaporated and contaminantshave been removed therefrom according to the present invention, thefluid may be condensed and recycled for use in the cleaning process. Forexample, FIG. 1 shows a collection vessel 17 into which gaseous HFC orHCFC fluid is pumped and condensed to liquid HFC or HCFC fluid. Theliquid HFC or HCFC fluid can be recycled back to container 11 for reuse.

EXAMPLE

[0029] In order that the invention may be more readily understood,reference is made to the following example which is intended to beillustrative of the invention, but it is not intended to be limiting inscope.

[0030] This example illustrates the efficacy of various HFC and HCFCfluids in cleaning applications.

[0031] Four HFC/HCFC fluids were prepared in accordance with the presentinvention and tested for their efficacy in cleaning circuit boards. Thefour fluids, comprising R-22, R-32, R-134a, and R-125 with 0.5 wt %methanol, respectively, were tested under various temperature andpressure conditions. The results of such tests are summarized inTable 1. Each of the fluids of the invention met or exceeded thecleaning efficacy of carbon dioxide, used as a standard cleaning fluidin industry. For comparison, the cleaning efficacy of carbon dioxide,R-125 (neat) and R-143a were tested and the results of these tests areshown in Table 1.

[0032] The HFC/HCFC and carbon dioxide fluids were all tested via thefollowing experimental method:

[0033] Experimental method. Two 2 inch×2.5 inch coupons were cut from acircuit board. Each coupon was cleaned and then coated on both sideswith fully activated solder flux [Kenco 1585 RA flux]. The averagecoating per coupon was 20 milligrams per square inch. Two coupons werethen hung on a rack. The rack was put into a high pressure vessel thathad been washed with water and isopropanol to remove any ioniccontaminants. The high pressure vessel was sealed and evacuated. Thecleaning fluid being evaluated was then loaded into the vessel using avacuum rack and wet ice or very small amounts of liquid nitrogen. Oncethe fluid was loaded, the entire vessel was put into a bath that hadbeen equilibrated at the temperature of the experiment. Alternatively,the high pressure vessel was wrapped with a heating blanket that wascontrolled by a thermostat set at the desired temperature and theexposure to heating was maintained for a time sufficient to achieve therequired temperature. The timing of the exposure was then started. Afterthe required time of exposure, the fluid was allowed to escape from thevessel and condensed in a collector vessel for disposal. The couponswere then removed from the vessel and their cleanliness measured with anAlpha Metals Omegameter 600 conductivity meter. The amount of flux lefton the board was measured by washing the board with isopropanol andmeasuring the conductivity of the isopropanol wash using theconductivity meter. The cleanliness data reported in Table 1 report thepercentage reduction in the amount of flux left on the board. TABLE 1Cleaning efficacy of various fluids PRESSURE TEMPERATURE, TIME % NUMBERFLUID [PSIA] ° C. (MINUTES) CLEAN C1 CO₂ 1300 40 45 <10 C2 R-125(neat)875 95 45 <10 C3 R-143a 825 95 45 <10 E1 R-22 775 100 45 100 E2 R-22 15125 45 100 E3 R-22 151 25 5 78 E4 R-32 1200 100 45 90 E5 R-32 875 80 4580 E6 R-32 707 70 45 65 E7 R-32 245 25 45 10 E8 R-134a 700 110 45 60 E9R-125/0.5 wt % 875 95 45 60 MeOH

[0034] As shown in Table 1, several HFC's and HCFC's dramaticallyoutperformed carbon dioxide in removing ionic solder flux resides fromtest coupons when such HFC and HCFC fluids were applied at supercriticaltemperatures and pressures. We further found that these same fluidsremoved ionics, though to a lesser degree, when applied at temperaturesand pressures slightly below critical. As was noted previously, HFC's,as a class, have heretofore been considered to be poor cleaningcompounds. Although it is known that, in general, solvency tends toincrease in the critical region, the degree of contaminant removalexhibited by the HFC's was surprising, especially with respect to ionicsremoval since this phenomenon is not exhibited by supercritical CO₂.

[0035] Having thus described certain particular embodiments of theinvention, various alterations, modifications and improvements willreadily occur to those skilled in the art. All such alterations,modifications and improvements are intended to be part of thisdescription though not expressly stated herein, and are intended to bewithin the spirit and scope of the invention. Accordingly, the foregoingdescription is by way of example only, and not limiting. The inventionis limited only as defined in the following claims and equivalentsthereto.

What is claimed is:
 1. A method for cleaning an article of manufacturecontaining contaminants on a surface thereof comprising the steps of:(a) providing a compound comprising 1,1,1,3,3-pentafluoropropane as asupercritical fluid; (b) contacting a surface of an article ofmanufacture containing contaminants with said supercritical fluid; and(c) removing substantially all of said supercritical fluid from saidsurface of the article of manufacture.
 2. The method of claim 1 whereinsaid supercritical fluid further comprises an oxygenate compoundselected from the group consisting of aldehydes, ketones, esters, ethersand alcohols.
 3. The method of claim 2 wherein said oxygenate compoundis an alcohol selected from the group consisting of methanol, ethanol,n-propanol, isopropanol, n-butanol, isobutanol and tert-butanol.
 4. Themethod of claim 1 wherein said supercritical fluid further comprisesfrom about 0.1 wt % to about 0.9 wt % of methanol.
 5. The method ofclaim 1 wherein said supercritical fluid is a fluid in its liquid state.6. The method of claim 5 wherein said supercritical fluid is at or belowabout the critical temperature of 1,1,1,3,3-pentafluoropropane.
 7. Themethod of claim 5 wherein said supercritical fluid is at or below aboutthe critical pressure of 1,1,1,3,3-pentafluoropropane.
 8. The method ofclaim 1 wherein said providing step (a) comprises providing saidcompound comprising 1,1,1,3,3-pentafluoropropane at a temperature offrom about −10° C. to about 200° C.
 9. The method of claim 8 whereinsaid providing step (a) comprises providing said compound comprising1,1,1,3,3-pentafluoropropane at a temperature of from about 15° C. toabout 130° C.
 10. The method of claim 8 wherein said providing step (a)comprises providing said compound comprising1,1,1,3,3-pentafluoropropane at a pressure of from about 25 psia toabout 10,000 psia.
 11. The method of claim 9 wherein said providing step(a) comprises providing said compound comprising1,1,1,3,3-pentafluoropropane at a pressure of from about 100 psia toabout 3,500 psia.
 12. The method of claim 1 wherein said contacting step(b) comprises covering at least a contaminated portion of said articlewith said supercritical fluid.
 13. The method of claim 12 wherein saidcontacting step (b) further comprises dissolving in said supercriticalfluid at least a portion of a contaminant on said contaminated portionof said article.
 14. The method of claim 1 wherein said contacting step(b) comprises immersing at least a contaminated portion of said articlein a stream of said supercritical fluid.
 15. The method of claim 1wherein said removing step (c) comprises converting said supercriticalfluid to a gaseous state.
 16. The method of claim 15 wherein said stepof converting said supercritical fluid to a gaseous state compriseschanging the pressure to which the supercritical fluid is subjected. 17.The method of claim 16 wherein said step of converting saidsupercritical fluid to a gaseous state further comprises changing thetemperature of the supercritical fluid.
 18. The method of claim 1wherein said article of manufacture comprises a printed circuit board.19. The method of claim 1 wherein said article of manufacture comprisesa silicon wafer.